Yes, ABSOLUTELY it will charge twice as fast.
But it goes the other way too, if u use the iphone charger on ipad it will charge 2 times slower.
Actually that's a myth.
I've done considerable testing with USB power meters and an Ammeter to verify that an iPhone will only use 1Amp of electricity no matter what charger you use. I've tested with over 18 different charger adapters with the same result. Even a 3.4 amp charger.... yep, the iPhone uses exactly 1 amp and only 1 amp.
So if you use a 10 watt charger (2 Amps at 5 volts) the iPhone will not charge faster. It will charge at the identical speed as if you used the included 5 watt (1 Amp) charger.
The reverse is certainly true. A 1 Amp (5 watt) charger can only deliver 1 Amp so the iPad will charge about 1/2 the speed.
Additional things to consider:
1) The iPhone (and iPad) will only rapid charge up to 80%. At 80% they both gradually reduce charging speed. The iPhone will use 1 amp up to 80%. Between 80%-100% it will use gradually decreasing power below 1 Amp.
2) The charger cable you use absolutely matters. Cheap 3rd party iPhone cables, especially long ones, use wire gauge that's too thin. As a result the iPhoen can use only a maximum of .6 amps, and in some cases .3 amps with a 10ft cheap-o that I tested.
It's worth paying for "apple certified" cables and avoiding the cheap $2 cable. All apple certified cables will carry a full 2.1 Amps. Those cheap ones will work but expect slow charge times (in some cases you may want that.)
3) Chargers need to say apple compatible otherwise you may be limited to the .5 Amp USB spec.
I used to believe that a 10 or 12 watt charger would charge the iPhone faster so I did some testing and proved I was wrong.